The SASE emission can be approximated as the scattering of the quasi-EM wave from the electron pulse in the EF. The emission takes part in the microbunching of the electron pulse, and is significantly enhanced at the fundamental frequency and its harmonics. In the lab frame, the fundamental frequency \(\omega _{\mathrm {fd}}=2\gamma ^2(2\pi /\lambda _u)/(1+K^2)\)  for the helical undulator, where \(K=eB_0/mk_u=ea/m\)  is the undulator parameter [1]}, [2]}, [3]}.
